Railway vehicles conventionally use rotating axle wheelsets in which the two flanged wheels are firmly attached to the axle and therefore are required by torque in the axle to turn at the same speed. Alternatively, rail vehicles can be equipped with wheelsets in which the wheels can rotate independently with little or no exchange of torque through the axle.
Both new and worn wheel treads typically provide a slightly larger rolling radius on the load carrying portion of the tread near the flange than on the portion of the tread which is remote from the flange and also further from the track centerline.
When the wheels are conventionally attached to a rotating axle by a rigid press fit, the wheelset has a self-steering property which will tend to steer the wheelset toward the centerline of tangent track when the wheelset is displaced laterally. This self-steering property will also provide steering toward the radial position in gradual curves. However, the self-steering property has the serious disadvantage that it tends to cause lateral oscillation of the wheelset with respect to the track centerline at high speeds. In addition, all railroads contain some curves and some railroads contain many curves in which the differential rail length is greater than can be accommodated by the differential radius of the two wheels in the set. In this case, the wheelset must be steered around the curve by a steering moment supplied by the truck framing. In sharp curves, the required steering moment becomes quite large.
An alternative wheelset configuration allows for independent rotation of the wheels with little or no torque exchanged through the axle. In some cases, the axle may not rotate at all. This type of wheelset has the advantage that it does not have a tendency to lateral oscillation even at very high speeds. However, the self-steering property of the conventional wheelset is also lost, and the wheelset must be steered by the truck framing at all times. In contrast to trucks having fixed wheel sets, the steering moment required is very small even in very sharp curves.
In one aspect, the present application is concerned with the adaptation of many features of the parent applications referred to above to trucks equipped with conventional rotating axle wheelsets. By virtue of such adaptation, it is possible to utilize features of the invention to retrofit existing railroad trucks as well as apply the invention to the design of new trucks.
In another aspect, the present application is concerned with utilizing features of the invention to provide axle steering to wheelsets having wheels which are able to rotate independently and therefore lack a significant self-steering ability.
The axle steering features of particular value to rail vehicles having independently rotatable wheels can also be applied to the field of highway vehicles which conventionally have independently rotatable wheels and where use of certain steering features of the invention can reduce lateral scrubbing of the tires and reduce the width of the roadway required for negotiating curves with long trailers.
Because the various aspects of my invention are especially useful in railway vehicles and particularly in railway trucks having a plurality of axles, the invention will be illustrated and described with specific reference to railway rolling stock.
The axles of nearly all of the railway trucks now in general use are rigidly constrained to remain substantially parallel at all times (viewed in plan). Passenger car and locomotive trucks conventionally also require the axles to be elements of a rectangle. Most of the freight car trucks now in general service do not constrain the wheelsets closely to the rectangular pattern and allow the wheelsets to run in a parallelogrammed position. In addition, the tolerances observed in freight car truck manufacture often do not provide adequate precision of the parallel position for low rolling resistance on tangent track.
Theoretically, a precisely parallel orientation of the wheelsets is sufficient for low rolling resistance at low speed on tangent track. However, this is not adequate for operation at the high speeds and high axle loads which are rapidly becoming commonplace around the world.
One undesirable result of allowing the axles to parallelogram is truck hunting. This leads to many undesirable and dangerous results such as lading damage, damage to the car structure and occasional derailments. The derailment hazard is due in part directly to the high wheel/rail forces present during hunting and indirectly to the cumulative track damage done by these forces.
Another undesirable result of restraining the axles to be parallel is having the lead axle run with a substantial angle of attack against the outer rail in curves, causing objectionable noise and excessive wear of both flanges and rails. This operation also presents a derailment hazard. The hazard is due in part to high flange climbing forces associated with the wheel/rail angle of attack and in part to the cumulative damage done to the track by the high forces.
Recent efforts by others to overcome the stability problem of conventional trucks have concentrated on restraining the parallel yaw motion of the two axles by restraining the yaw motion of the truck bolster relative to the vehicle. This is done by means of constant contact side bearings which apply a substantial friction force longitudinally between the car body and the bolster at a location approximately two feet removed from the point of truck swivel. While this measure will provide some suppression of truck hunting, curving is made worse, and there is usually a noticeable increase in flange wear. In addition, the service life of constant contact side bearings is relatively short. This is in contrast to trucks of this invention which have a very long service life and require very little maintenance.
Another approach to the hunting problem has been the introduction of devices for rigidizing the truck frame to prevent parallelogramming of the wheelsets. Tests have shown that truck hunting is also suppressed, but again, curving is made worse.
A third approach to the hunting problem has involved rigidizing the truck frame plus the use of resilient pads between the truck framing and the wheelsets. This will allow a limited measure of self-steering of conventional rotating axle wheelsets. However, some of these designs provide such limited suppression of truck hunting that constant contact side bearings are still required. On the other hand, one truck frame rigidizing design described and claimed in my U.S. Pat. No. 4,483,253 has proven to be relatively successful without requiring constant contact side bearings. As a result, this configuration has a long service life. However, the suppression of truck hunting is still not as effective as with the present invention, and the improvement in curving has a more limited range.
For the purposes of this disclosure, the term "yaw" stiffness is defined as the restraint of the wheelsets relative to the truck framing in the yaw direction. In the apparatus of the invention, yaw stiffness is provided in part by the elastomeric shear pads and in part by direct elastic connections between the two steering arms and elastic connections between one of the axles and the car body which may involve connections between the car body and the truck framing. When rotating axle wheelsets are used, the value of yaw stiffness required to control the truck hunting must be relatively high. In some applications where the yaw stiffness is provided by the shearing action of load carrying pads, it is often desirable to limit the yaw forces by means of sliding surfaces employing material having a carefully selected friction characteristic. An alternative method for providing a high stiffness for small motions and a lower stiffness for large motions is the use of non-linear springs, particularly between the two axles. Another means for providing the required yaw stiffness is a longitudinal member interconnecting one axle, the truck framing and the vehicle body in such a way as to create yaw moments which restrain deviations from a radial position in curves and from the parallel position on straight track. This longitudinal member, called a tow bar, can provide other desirable characteristics as described later.
The term "lateral" stiffness is defined as the restraint of one wheelset of a pair relative to the other in the direction paralleling the general axis of rotation. In the apparatus of the invention, the lateral stiffness acts to restrain parallelogramming of the wheelsets, and this stiffness is provided in part by the stiffness of the steering arms and in part by the stiffness of the elastomeric coupling means between the two arms.
Two major objectives of this invention are to prevent hunting and to improve the curving of trucks equipped with conventional self-steering rotating axle wheelsets, in part by applying lateral stiffnesses directly between the axles through the use of steering arms and in part by providing carefully chosen yaw stiffnesses of the axles relative to the truck framing and the car body.
In addition, I have discovered that similar means can be used to provide axle steering for rail vehicles having wheelsets in which the two wheels on each axle are free to rotate independently.
To achieve these general purposes, and with particular reference to railway trucks, the invention provides an articulated truck so constructed that: (a) steering arms directly interconnect pairs of axles to provide for exchanging steering moments between the two axles without involving the main truck framing; (b) carefully chosen values of yaw stiffness are provided relative to the truck framing and the other axles which tend to return the axles to a parallel position; (c) supplementary values of yaw stiffness may be provided between the truck and the vehicle where needed; and (d) non-linear values of yaw stiffness may be provided between the steering arms.
A retrofit embodiment of the invention applied to an existing conventional truck using conventional rotating axle wheelsets has been tested successfully at more than 90 miles per hour with virtually no trace of instability. This is in contrast with conventional trucks which are usually unstable at speeds above 45 miles per hour. A group of cars equipped with this embodiment has been found to roll as easily in a 4.degree. curve as on straight track. This is in contrast to trains on conventional trucks which begin showing additional rolling resistance in curves sharper than 1.degree.. In addition, the rolling resistance of conventional trucks in sharp curves is several times larger than the rolling resistance of cars retrofitted with the apparatus of this invention.
An embodiment consisting of a new truck with steering arms and tow bar steering was tested utilizing independently rotatable wheels. The tendency to truck hunting was found to be completely eliminated. Quiet curving was achieved in a curve of less than 50 foot radius with almost no increase in rolling resistance. Another embodiment having independently rotatable wheels and employing steering arms with carefully chosen yaw stiffnesses between the truck framing and the car body was found to give similar results.
In many of the tow bar arrangements, the tow bar elements handle longitudinal forces between the car body and the steering arms or sub-trucks, thereby taking care of forces arising, for example from coupling impacts, propulsion, and braking.
The invention further contemplates the use of the tow bar linkage to provide steering for rotatable axle wheelsets and increase the high speed stability of conventional rotating axle wheelsets.
One embodiment uses two tow bars laterally displaced from the vehicle centerline which share longitudinal restraint of the truck framing and steering arms relative to the vehicle, one of said tow bars acting as a steering linkage, pivotally exchanging lateral steering forces among one steering arm, the truck framing and the car body. This construction is utilized to avoid mechanical interferences with other essential truck parts.
To more fully describe the stabilizing influence of the tow bar steering feature of the invention, it is necessary to consider the deviations of vehicle speed from the "Balance Speed" which is defined as that speed on a banked curved track at which there is no net lateral force relative to the track. It is a general practice to bank railroad track so that the Balance Speed is close to the normal operating speed. Above the Balance Speed, there is an outward net centrifugal force. Below the Balance Speed, there is a net force toward the center of the curve. It is important to understand that nearly all rail vehicles have some form of lateral suspension flexibility which permits some variation in the lateral position of the car body relative to the center of the track in the direction of the net lateral force. It is also important to recall that the net lateral forces due to curvature and speed are usually small compared to the lateral wheel/rail forces generated by wheelsets whose axles are not in a radial position.
One the objects of the tow bar apparatus is to modify the yaw position of the wheelsets in response to lateral motion of the car body relative to the wheelsets in such a direction as to enhance stability and safety. This modification is analogous to the understeer characteristic built into highway vehicles for the same purpose.
On tangent track, the effect of a lateral wind gust is to cause the wheels to more easily move toward the lee rail. This will prevent cross wind forces from creating lateral instability of the wheelsets and car body. Experimental evidence collected from certain earlier steerable axle truck designs which attempted to utilize centrifugal forces to cause the car to steer into curves and also caused the wheelsets to steering into the wind were found to be relatively unstable on tangent track.
When operating in curves, the tow bar apparatus of the invention will urge the wheelsets toward the outer rail when the vehicle is running above the Balance Speed and toward the inner rail when running below the Balance Speed. This relationship may at first seem counterintuitive. But wheel climbing derailments do not occur when vehicles operate above the Balance Speed. Above the Balance Speed, the vertical force on the outer rail also increases, preventing the flange climbing tendency that might be expected. On the other hand, derailments frequently occur when operating below the Balance Speed. This is due to the fact that, with conventional trucks, there is always a large outward flange force on the outer rail, but below the Balance Speed the vertical load on the outer rail is reduced, and flange climbing is therefore made easier. With the tow bar feature of the invention, the wheelset is steered toward the inner rail where the vertical load is also increased, and flange climbing cannot occur.
My invention also provides improved lateral brake shoe guiding which will virtually eliminate contact of the brake shoes with the wheel flanges. The uneven wear of wheel flanges associated with conventional brake beam support methods tends to cause a wheel diameter mismatch in conventional rotating axle wheelsets and this shortens wheel life. The invention also contemplates improvements to the support of the brake shoe when handling braking forces. This improvement will compensate for the wheel unloading associated with the longitudinal braking force created between the vehicle and the track, lessening the tendency for generating flat wheels.